A typical catalytic converter comprises a cylindrical catalyst substrate mounted within a cylindrical catalytic converter housing. The catalyst substrate may be a ceramic honeycomb or a corrugated metal foil sheet and a flat metal foil sheet wound together into a spiral defining a matrix of passages.
In either case the catalyst substrate defines a multiplicity of the flow passages extending therethrough generally parallel to an axis of the catalyst substrate and, when installed therein, the catalytic converter housing.
Catalytic converters may be broadly grouped into vehicle sized units and stationary engine or industrial sized units. Vehicle sized units are considerably smaller than industrial sized ones and accordingly are relatively easy to remove and to disassemble. Catalyst substrate diameters for vehicle sized units would typically measure less than a foot (approximately 0.3 m). In contrast, large industrial sized units may have catalyst substrate diameters that measure on the order of six feet (approximately 2 m). The associated ducting and sheer size of the components typically precludes removal and axial disassembly of an industrial sized unit for replacing the catalyst substrate. Instead, large industrial sized catalytic converter housings are provided with a lateral access port for removal of the catalyst substrate from a side of the housing without removal or axial separation of the housing from its associated ductwork.
In use the catalytic converter housing both during heat up and steady state operation will typically be about 100° C. cooler than the catalyst substrate. This is because the substrate typically runs at exhaust temperature and has nowhere to conduct or radiate heat away. The housing in contrast will receive heat from its inside but can radiate or conduct heat into the surrounding atmosphere. Upon shutdown or low engine loads, the rate of the temperature loss from the housing tends to be less than that of the catalyst substrate because the housing is of heavy gauge metal whereas the catalyst substrate is thin sheet metal with a huge surface area. The housing under low engine load conditions can be 100-150° C. hotter than the catalyst substrate.
Considering the overall size of an industrial sized unit, the temperature differential can result in significant dimensional differences between the housing and the substrate. These must be accommodated to avoid undue stress damaging either component while ensuring adequate sealing therebetween so as to avoid exhaust gasses escaping between the housing and the substrate.
Most large industrial catalytic converters are sealed about the periphery of the catalyst substrate with a ceramic fibre material. Unfortunately such material is prone to erosion by high velocity gasses and mechanical breakdown through compression and vibration. Furthermore such material is easily torn and difficult to maintain in place during installation, particularly with larger units.
It is an object of the present invention to provide a catalytic converter design which allows for differential thermal expansion between the catalytic converter housing and the catalyst substrate without the use of fibrous gasketing materials yet ensuring an effective seal to avoid excessive gas flow between the substrate and the housing.